This invention relates to apparatus for controlling a plasticizing process of an in-line screw-type injection molding machine so as to obtain an injection molded product having uniform quality by uniformly controlling the temperature of a material such as resins to be molded.
In a prior art injection molding process using an in-line screw-type injection molding machine, there have been proposed various methods and apparatus to obtain an injection molded product having uniform quality by controlling a plasticizing process by changing the number of revolutions and/or back pressure of the screw of an injection molding machine.
However, in a prior art injection molding machine, the temperature and the viscosity of the melted material such as resins are not controlled or controlled by presetting programs regarding the number of revolutions and/or the back pressure of the screw after the completion of the plasticizing process and it has been difficult to obtain a molded product having highly uniform quality, for example, in view of the fact that melted resin prepared in one plasticizing cycle is not always used for one injection process. The non-uniform quality of the product may result in cracks and spoil the appearance of the product.
Thus, in order to obtain an injection molded product having uniform quality, it is necessary to strictly control or amend such factors, in the plasticizing or injection process, as heating temperature, shearing energy caused by the rotation of the screw, back pressure and the rotating speed of the screw for the reasons described in detail hereunder.
Generally, in an in-line screw-type injection molding machine, a material, usually a resin, supplied into a heating chamber from a hopper is melted by heating energy generated by a heater and shearing energy caused by the rotation of the screw arranged in a heating chamber.
The heating energy is applied to the resin in proportion to the time when the resin exists in the heating cylinder, and the heating time is determined on the basis of the correlation between all amount of the resin B' existing in the screw threads at a time when the screw moves backwardly by a distance of a measuring stroke necessary for the molding after the completion of a measuring process and an amount of resin A' necessary for manufacturing one molded product. A ratio B'/A' is generally equal to N'+.alpha. (N': integer; 0&lt;.alpha.&lt;1). In this equation, where .alpha.=0, all of the raw material resin supplied into the heating cylinder in one plasticizing process is injected after N' molding cycles (this is the case where the amount of the resin in a portion of the heating cylinder corresponding to "5a" in FIG. 4 equals to zero), and the resin to be injected by one injection process from the heating cylinder is subjected to the uniform heat energy in the cylinder. In this case, there is no problem, but a case wherein .alpha.=0 is considerably rare and where .alpha..noteq.0, a certain amount of resin remains in the hopper after N' shots and this remaining resin will be mixed with a material resin fed by the next measuring process. The resin fed into a space defined at the front end of the heating cylinder consists of two portions. One portion is a portion of the resin fed into the heating cylinder in the former molding cycle (5a in FIG. 4) and the other is a portion of the resin fed therein in the next molding cycle (5b in FIG. 4). Thus, there exist resins having different temperatures and/or viscosities in the heating cylinder, and a molded product made of these resins has such defects as forming of flow marks, roughness of the surface and non-uniform strength.
From another point of view, the resin supplied from the hopper is melted in the heating cylinder by the heat energy generated by a heater and the shearing energy caused by the rotation of the screw. The shearing energy is increased in proportion to the effective length of the screw at a time when the resin which has already been supplied from the hopper into the space between the screw threads is fed into a space formed at the front end of the heating cylinder to temporarily accumulate the melted resin before the injection process. The effective screw length momentarily varies for the reason that the screw is backwardly moved during the resin measuring process. Therefore, even the resin supplied in one measuring process is subjected to shearing energies which are different at the various portions of the resin and the temperature of the resin fed into the space formed at the front end of the heating cylinder is not uniform.
From still another point of view, the temperature of the melted resin is generally increased by applying pressure, and accordingly, in an in-line screw-type injection molding machine, the temperature of the resin between the screw threads is caused to change by the increase of the pressure in a process of filling the resin into the mold cavity. Therefore, even if the temperature of the resin in the heating cylinder after the plasticizing process were uniform, the temperature would become non-uniform in the subsequent filling process by the increase of the pressure. Thus, the resin to be injected has a non-uniform distribution of temperature.
From the other point of view, the temperature of the melted resin in the filling process is caused to change by the change of the shearing energy caused by the change of the flow speed of the melted resin. The change of the shearing energy cannot be ignored because there are some members having relatively narrow cross-section such as a nozzle, a runner and a gate in the flow path of the resin so that the shearing energy is affected by these members.
For example, FIG. 1 shows a graph representing the relation between the injection ratio and the temperature ratio of a resin such as polyethylene or polystyrene molded through a nozzle having an inner diameter of 3 mm. In the graph, the temperature ratio "1" shows the temperature in a case where the injection ratio is 100 cm.sup.3 /sec. The correlation between the injection ratio, i.e., the filling speed, namely the shearing energy, and the temperature ratio can be understood from this graph.
Particularly, in an injection molding machine in which a program of the resin filling speed is controlled, the flow speed is positively varied in the filling process, so that the shearing energy generated during this process largely varies and the resin filled in the mold cavity has a temperature distribution in proportion to the resin filling speed. Therefore, even if the temperature of the resin plasticized in the heating cylinder were uniformly controlled after the measuring process has been completed, the temperature of the resin would become non-uniform by the effect of the shearing energy during the filling process after the measuring process for the reason described before. However, the program control of the filling speed of the melted resin which results in the variation of the temperature of the resin must be carried out so that the flow of the resin would not be uniform in the mold cavity. Particularly, since the mold cavity comprises portions having relatively large and small cross-sections and the flow speeds of the resin are different at respective portions, the flow speed is program controlled so that the resin would flow at a high speed at a portion having large cross-section and at a low speed at a portion having small cross-section. However, in view of the shearing energy at the nozzle portion, a molded product will have a higher temperature in thick portions and a low temperature in thin portions. Thus, when the molded product is cooled, the temperature distribution further varies largely thus damaging the molded product.